This invention relates to a torque motor and, more particularly to a torque motor which may be used for a flow control valve of an internal combustion engine, for instance.
As disclosed in JP-A 3-31529 and JP-A 6-253516, a conventional torque motor uses permanent magnets shaped arcuately as a part of its rotor. Those torque motors may be used as an actuator for an intake air flow control valve of an internal combustion engine. However, manufacturing the permanent magnets into the arcuate shape necessitates a cutting process and produces waste of magnetic material.
It is therefore proposed that a torque motor uses a plurality of plate-shaped permanent magnets on its outer periphery. That is, as shown in FIG. 17, a torque motor has a rotor 41 comprising a rotor core 42 and two diametrically opposed permanent magnet groups 43 and 44 arranged symmetrically with respect to a rotary axis of the rotor 41. The permanent magnet groups 43 and 44 are spaced apart from each other on the rotor core 42 in the circumferential direction. The magnet group 43 has a plurality of plate-shaped permanent magnets 43a arranged in the circumferential direction on a part of the outer periphery of the rotor core 42, while the magnet group 44 has the same number of plate-shaped permanent magnets 44a arranged in the similar manner on another part of the rotor core 42. The stator cores 45 and 46 are joined at a pair of stator core connecting parts 48 and 49. The connecting parts 48 and 49 for the corresponding magnet groups 48 and 49 are diametrically opposed on the circumference of the rotor core 42. The rotor 41 is held rotatably inside of a pair of stator cores 45 and 46 between which solenoid units 50 and 55 are sandwiched.
The above torque motor generates a torque varying in dependence on the rotational angle of the rotor as shown in FIG. 18. The torque is a composite of torque generated most strongly at two locations, that is, at an X-part near the slot connecting part 48 and a Y-part near the slot connecting part 49. As the magnet groups 43 and 44 are arranged symmetrically with respect to the rotary axis of the rotor 41 and are spaced apart from each other circumferentially on the rotor core 42, the torque fluctuates periodically during rotation of the rotor 41 whether the solenoid units 50 and 55 are energized or not energized, thus resulting in ripple on the generated torque.
It is also proposed by JP-A 9-84283, JP-A 9-84284 and JP-A 9-93844 that a torque motor have a non-magnetic cylindrical cover fitted to firmly hold permanent magnets on the rotor. In this instance also, the torque generated by the motor fluctuates periodically.